In multi-user wireless communication systems, such as mobile phone networks, wireless local area networks, and satellite communications, multiple transmitters and receivers may communicate simultaneously through a common wireless communication medium. One communication format widely used by multi-user systems is Code Division Multiple Access (CDMA), in which the transmitters generate orthogonal waveforms that can be separated by the receivers thereby enabling simultaneous transmissions from multiple users over the same time-bandwidth slot. More specifically, each transmitter applies one code chosen from a set of orthogonal “spreading codes” to an outbound serial stream of “symbols.” Each symbol represents a discrete information bearing value selected from a finite set (“alphabet”). For example, simple alphabets used by transmitters may be {+1,−1} or {−3,−1,+1,+3}. The application of the orthogonal spreading codes to the symbols produces a set of “chips” for each symbol to be transmitted. The resulting chips are transmitted according to some modulation scheme, such as quadrature phase shift keying (QPSK) modulation. In order to separate signals from multiple users, the receivers isolate the signal of the desired user by matching the signal to the corresponding orthogonal spreading code.
When the transmission rate increases, the communication medium can become “frequency selective” in that certain frequencies exhibit significant fading, i.e., significant loss of signal. This property often causes inter-chip interference (ICI) in which the transmitted chips for a particular symbol interfere with each other, destroying the orthogonality of the waveforms at the receiver. By rendering the transmitted waveforms non-orthogonal, ICI can lead to multiple user interference (MUI), in which the receivers are unable to correctly separate the waveforms, eventually leading to data loss and/or bandwidth and power inefficiencies. In addition to intra-cell interferences, inter-cell interference also arises from the transmission of waveforms from nearby base stations. Inter-cell interference is most severe when a user is at the edge of a cell. In CDMA wireless communication systems, soft handoffs are employed to allow a mobile station to communicate with multiple base stations simultaneously, improving the transmission quality of the wireless communication medium and avoiding disconnection upon base station switching. Soft handoff techniques substantially reduce the ping-pong effect when the mobile user is on the edge of two cells, and has to switch between two base stations frequently. In the soft handoff mode, the same information block of the desired user is transmitted simultaneously from all candidate base stations.
Various techniques have been developed that attempt to suppress the effects of MUI. For example, various linear and non-linear “multi-user detectors” have been developed for separating non-orthogonal user waveforms. These detectors, however, typically use techniques that require knowledge of the characteristics of the current communication medium and that are often complex and expensive to implement in typical mobile communication devices. As a result, these detectors are more suitable for uplink transmissions, where the base station has knowledge of the multipath channels and spreading codes of all users, and is thus able to demodulate all users' information either jointly, or, separately. In addition, alternatives to CDMA have been proposed including multicarrier (MC) spread spectrum based multiple access, e.g., (generalized) MC-CDMA and Orthogonal Frequency Division Multiple Access (OFDMA), where complex exponentials are used as information-bearing carriers to maintain orthogonality in the presence of frequency selective channels. Multicarrier schemes are power inefficient because their transmissions have non-constant magnitude in general, which causes power amplifiers to operate inefficiently. These alternatives can also be very complex and expensive to implement and do not necessarily compensate for channels that introduce significant fading.